The present invention relates to a driving apparatus for a photoreceptor of an image forming apparatus in which an image is formed by a digital method in an electrophotographic method, and specifically relates to an image forming apparatus in which a plurality of developing devices are arranged around the photoreceptor, and a color toner image is formed by superimposing a plurality of mono-color images on each other, which are developed by the developing devices.
In copiers or printers using the electrophotographic method, an image is obtained by the following operations: a cylindrical or belt-shaped photoreceptor is rotated; an electrostatic latent image is formed on the surface of the rotating photoreceptor; toners are adhered onto the latent image during development and a toner image is formed; the toner image is transferred onto a transfer material; and the image on the transfer material is fixed.
In the above image forming apparatus, when fluctuations in the speed of the rotating photoreceptor are caused for any reason, jittering occurs and unevenness results on the outputted image. This problem is conspicuous in the digital type electrophotographic technology in which image data is written by scanning of a semiconductor laser. Speed fluctuations in the rotation of the photoreceptor cause speed fluctuations in the data writing system in the subsidiary scanning direction, and thereby, minute displacement is caused in the interval between written lines, resulting in severely lowered image quality.
Specifically, in a color image forming apparatus, in which a plurality of developing devices are arranged around the photoreceptor, and a color toner image is formed when a plurality of mono-color images are superimposed on each other, the color tone of the secondary color, which is formed by more than 2 toner layers, is determined by an appropriate adhered amount of each toner of each layer. Accordingly, accuracy of the position of each layer is essentially required. That is, in the image forming process after the second toner layer, exposure by the semiconductor laser, or the like, is carried out from above the toner layer which is already formed on the photoreceptor. Therefore, although the exposure should be carried out onto the toner layer, the surface of the photoreceptor, on which no toner layer yet exists, is exposed when the position to be exposed is displaced by the adverse influence caused by speed fluctuations of the photoreceptor. At this time, the absolute value of the surface potential of the photoreceptor is lowered, compared with the case where the toner layer already exists. Accordingly, the adhered amount of toner after development is increased more than in the case where the exposure is carried out from above the toner layer. As a result, the color tone in this portion deviates from the desired value. In many cases, in digital type image output apparatus, an image is composed of lines which are formed of a series of dots. In this case, a difference in the latent image forming process is caused between the portion in which the interval of lines to be exposed is decreased by speed fluctuations of the photoreceptor, and the portion in which the interval of lines to be exposed is increased. Accordingly, the electric potential distributions on the photoreceptor are different from each other, resulting in large differences between toner adhered amounts. This causes uneven color of the image in the rotational direction of the photoreceptor. The color image quality is severely lowered for the this reason.
On the other hand, in the conventional designs of drive systems in copiers or printers, it is a major object to determine an appropriate position of the subject to be driven, considering available space, while satisfying the line speed, or the number of rotations which are derived from specifications of the product. That is, the following are a matter of large concern: by what method is the mechanical power supplied from the power source to be transmitted to the subject to be driven; or what mechanical components are to be selected for power transmission. Accordingly, when jittering or unevenness due to rotation occurs in the final product, the causes of unevenness are investigated, and the following countermeasures are considered: the drive shaft bearing of the photoreceptor is replaced with a sintered one; a flywheel is connected to the drive shaft of the photoreceptor; a brake, including a spring and a friction member, is attached onto the rotational shaft of the photoreceptor; accuracy of gears is increased; or helical gears having various twist angles are used.
As described above, it is essential to increase the driving accuracy of the photoreceptor drum in order to increase the image quality in the digital image formation.
There are various factors by which the driving accuracy of the photoreceptor is deteriorated. As the above-described factors relating to the driving system of the photoreceptor, which mainly control the drive of the photoreceptor itself, the following factors are described: driving unevenness of the main motor; eccentricity of the motor shaft; and a specific frequency component of the specific structure of the motor. Concerning the gear train in the driving system, the fluctuations in one tooth or one rotation of gear caused by inaccuracy of gears, specifically, the accuracy of the tooth form, the accuracy of the tooth trace, the eccentricity of the gear, or the like, are listed. Further, also in the case where the driving system is composed of a motor and gears, the positional relationship of the motor shaft and gear shafts, specifically, the distance between shafts directly affects an amount of backlash of gears, and errors in shaft alignment cause vibrations at points of engagement between gears.
The driving accuracy of the photoreceptor greatly affects not only the driving system, but also the case where peripheral units are operated. One of the units which have a major affect on the system, is the developing unit. The developing unit has a moving portion for rotating the developing sleeve and the mixing screw at relatively high speed, and has a driving system for driving the moving portion independently or in combination with other units. In cases where the distance between the developing unit and the photoreceptor is determined by applying a member (a roller) to the surface of the photoreceptor, the vibrations generated in this driving system are directly transmitted to the photoreceptor through this member, and adversely affect the driving accuracy of the photoreceptor. Further, in also a development driving system, vibrations are generated depending on engagement conditions of the developing unit with a coupling portion of the driving system. This phenomena is due to the amount of backlash of coupled gears and mis-alignment of the driving shaft. The transmission ratio of the vibrations, generated in the developing unit and transmitted to the photoreceptor, changes depending on a supporting method of the developing unit. When the unit is fairly firmly supported, the degree of vibration transmitted to the photoreceptor is small. When the unit is inadequately supported, the unit itself vibrates at that position, and vibrations at each portion are transmitted to the photoreceptor without attenuation.
Moving portions are provided in not only the developing unit, but also in the cleaning unit, the transfer unit and the conveyance unit, or the transfer and conveyance unit which is integrally composed of the transfer unit and the conveyance unit. Accordingly, vibrations are inevitably generated. These vibrations are transmitted through the photoreceptor cartridge or the frame of the photoreceptor, which are provided near the above-described moving portions, and further, through the main body frame to the photoreceptor. Fluctuations of frequencies of vibrations are transmitted to the photoreceptor or the driving system of the photoreceptor, resulting in reduction of the driving accuracy of the photoreceptor.
As described above, when deterioration of the driving accuracy of the photoreceptor is generated, displacement of lines is generated, and changes in image density and color unevenness are thereby caused.
The above-described problems have been solved or improved in the present invention. An object of the present invention is to provide an image forming apparatus in which the speed fluctuation of the photoreceptor is effectively reduced, and thereby, the photoreceptor is always driven at a stable rotation speed, when a very simple device is provided in the apparatus.